1. Field of the Invention
The present invention relates to a silica optical waveguide having a clad layer and a core formed of a silica material and a method of fabricating the same.
2. Description of the Related Art
In general, an optical waveguide has a substrate, a clad layer formed on the substrate and a core (optical waveguide) which is embedded in the clad layer and which has a refractive index slightly higher than that of the clad layer. The clad layer is constituted by a lower clad layer disposed under the core (on the substrate side of the core) and an upper clad layer disposed above the lower clad layer so as to embed the core. Light input to the optical waveguide is confined in the core because of the difference between the refractive indices of the core and the clad layer and is propagated in the core.
In a silica optical waveguide, silicon or silica is used as a material of the substrate thereof, and the core and clad layer are made of silicate glass using a flame hydrolysis deposition (FHD) process, sputtering process, or chemical vapor deposition (CVD) process. Since the refractive index of a silicon substrate is greatly different from the refractive index of the clad layer and the core formed of silicate glass, the position of the core formed on the silicon substrate can be relatively easily recognized when observed in a direction normal to the surface of the substrate. However, when the substrate is formed of a silica material, since the refractive index of the substrate and the refractive index of the clad layer and the core are substantially equal to each other, a problem arises in that the position of the core is difficult to recognize when observed in a direction normal to the surface of the substrate. This results in another problem in that an aligning and connecting operation of input/out fiber becomes difficult to perform properly.
When the material to form the upper clad layer is deposited such that it follows the shape of the core, a step is formed on the surface of the upper clad layer because of the presence of the core. It is therefore possible to recognize the position of the core easily by taking advantage of a difference in reflection between the flat portion and the step portion.
However, when there is a narrow gap region in a waveguide layout of cores on which an upper clad layer is deposited using a CVD process so as to embed the core, the material of the upper clad layer may not be deposited at the gap to leave a void (whisker) in such a region. Methods proposed to avoid this include a method in which an upper clad layer is formed while sputter-etching the same and a method in which an upper clad layer is formed using a material having a low softening temperature and is made to flow by softening it at a high temperature to fill a void (see Patent Document 1).
Patent Document 1: Specification of U.S. Pat. No. 6,044,192
However, when the generation of a void is suppressed by softening an upper clad layer to make it flow, the surface of the upper clad layer is softened and flattened, and a step on the surface becomes gentler, which results in a problem in that it will be difficult to recognize the position of a core.
It is an object of the invention to provide a silica optical waveguide in which the position of a core thereof can be easily recognized and a method of manufacturing the same.